Radio receiver selectivity

- overview or tutorial about the basics of radio receiver selectivity including filter performance and specifications, shape factor, crystal filters, LC filters, and ceramic filters.

Selectivity is one of the major specifications of any radio receiver for whatever application.

There are two main functions of any receiver - one is to amplify the signal to the required level, and the other is to separate it from the other unwanted signals. These two requirements are the two major elements for any radio receiver.

In view of this the selectivity performance and specification are form one of the major requirements for any radio receiver.


Receiver topology and selectivity

There is a variety of different radio receiver topologies that can be sued. The actual receiver technology can have a significant impact on the way. Although there are several types of receiver that can be used, the major types are summarised below:

  • Superheterodyne receiver:   The superheterodyne radio receiver has been in widespread use for many years, and it is still widely used for many high performance applications as well as for broadcast, television, communications and others.

    Note on the superheterodyne receiver topolgy:

    The superheterodyne radio receiver topology operates by changing the frequency of the incoming signal down to a fixed frequency intermediate stage where it can be amplified and filtered. A variable local oscillator signal is mixed with the incoming RF signal to achieve what is effectively a variable frequency filter.

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    A variety of selectivity and filter requirements are applicable for superheterodyne receivers. Selectivity of the front end is required to ensure sufficient image rejection, and the filters in the IF provide the main adjacent channel rejection.
  • Direct conversion receiver:   The direct conversion receiver also uses a mixer, but rather than converting the signal down to a fixed frequency intermediate frequency, it converts it down to baseband. Typically these receivers are used for data, where the baseband signal is applied to IQ demodulators and passed into a digital signal processing system. For these receivers the main form of filtering required is at the front end to prevent unwanted out-of-band signals entering the mixer and overloading the RF and later stages.

While there are other forms of radio receiver topology, these two encompass the majority of receiver designs.


Receiver selectivity

Selectivity is required in different areas of a radio receiver to ensure that only the wanted signal is received. Front end tuning or selectivity tends to be wideband in nature, or variable to enable the receiver to cover different channels or frequencies.

For the superheterodyne receiver, the main selectivity is provided within the fixed frequency IF stages. Here filters are able to provide very high degrees of selectivity ranging from that required for wideband transmissions including wideband FM, direct sequence spread spectrum and the like, occupying bandwidths of hundreds of kilohertz or megahertz, down to very narrow band transmissions occupying a few kilohertz or even a few Hertz.

Dependent upon the type of transmission the receiver will be used for, the bandwidth required, and hence its selectivity, different types of filter may be used.

Selectivity is a particularly important parameter in any radio receiver whether it is used for broadcast reception or for use within another form of radio communications system such as a two way radio communications link, or a fixed or mobile radio communications application. As a result it is necessary to ensure that any radio receiver is able to select the wanted signal as well as it can. Obviously when signals occupy the same frequency there is little that can be done, but by having a good filter it is possible to ensure that you have the best chance or receiving and being able to copy the signal that is required.

By Ian Poole


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